I recently built up a bike for a friend with a tapered frame and straight steerer fork.
Tapered fork to non tapered frame adapter.
I got a giant integrated tapered 1 1 8 1 1 2 headset from the lbs which also had the straight steerer crown race adaptor in it.
This will work with any bike with a pressed bottom cup that is 1 1 8 although i believe this no longer allows for frame warranty check with bike manufactur.
Traditional 1 1 8 in headtubes or 34mm inside diameter headtubes cannot accept a tapered steering column fork therefore we do not make an adapter that will allow you to use a tapered steering column on a traditional 1 1 8 in headtube.
If your mountain bike came with a straight steerer tube you can use this bearing adapter to put in a tapered steering tube and run a different fork.